[flang] Fix folding of ac-implied-do indices in structure c'tors

Array constructors with implied DO loops that oversee structure
constructors were being prematurely folded into invalid constants
containing symbolic references to the ac-implied-do indices,
because they are indeed "constant expressions" as that term is
used in the Fortran standard and implemented as IsConstantExpr().
What's actually needed in structure constructor folding is a
test for actual constant values, which is what results from
folding them later with repetition in the context of folding
an ac-implied-do.

Differential Revision: https://reviews.llvm.org/D115470